Current Optical Performance and Emissivity

Current Optical Performance and Emissivity

Intrinsic Optical Quality

Active figure and alignment control are in routine use (see wavefront sensing). The new secondary was installed on 14 June 1999; as noted above it displays none of the defects of its predecessor. Typically the overall wavefront error is now around 180 nm RMS, a factor 2 smaller than the typical values with the old secondary and ~1.3x worse than the formal “diffraction limit” at 2.0 microns. The new secondary has improved the nominal limiting Strehl ratio of the telescope from ~30% (somewhat better than the Upgrades Programme requirement of 25%) to ~73% (not far short of the Upgrades’ goal of the conventional definition of the diffraction limit, 80% Strehl). 

Delivered Optical Performance

Between 11 February 1998 and 29 September 1998 we accumulated 143 measurements of the image FWHM, determined in a standardized way and corrected for undersampling. Over this period the formal median K-band image FWHM was 0.”433, the best directly measured image FWHM was 0.”171 while about 10% of the images, after correction for undersampling, appear to have FWHM <0.”15.

Full details are given elsewhere .

Telescope Emissivity

The importance of emissivity and the expected value for UKIRT.

When working background-limited in the thermal IR the time required for an observation is proportional to the background signal. This signal is normally dominated by the telescope emissivity, which is thus an important factor in overall performance at these wavelengths (longer than ~2.3 microns). When coatings are fresh the telescope emissivity model predicts:

Primary Mirror (Al)                                             3.0%
Secondary Mirror (Al) 3.0%
Tertiary dichroic mirror (Ag - dielectric coating) 2.0%
Secondary central bevel (scattered environmental radiation) 0.2%
Secondary support vanes: - reflected in primary 1.0%
- direct view 0.0%
TOTAL: 9.2%

Currently the secondary vanes carry on their lower edges a strip of black NEXTEL material which absorbs incident light, so that changes in ambient light levels are not reflected from the vanes to the instrument, to which they are visible by reflection in the primary and secondary mirrors. (Also directly, in an annulus around the secondary, if the cold pupil stop is oversized: all current UKIRT facility instruments have cold pupil stops which are undersized relative to the image of the secondary in the pupil plane, so that the detectors have no direct view of the vanes. This is not necessarily an optimum solution, however.)

Future plans

In the near future the NEXTEL strips will be replaced by sky-reflecting “knife edge” baffles. This should reduce the effective emissivity of the vanes to ~0.1% Other improvements are planned in the longer term. A low-emissivity (overcoated silver) coating on the secondary (and perhaps eventually the primary) should reduce the surface emissivity to ~1% or even below. Such a coating on the tertiary is also possible if offset guiding for all objects is accepted. The combination of all these steps could reduce the overall emissivity below 4%:

Primary Mirror (overcoated Ag)                                  1.0%
Secondary Mirror (overcoated Ag) 1.0%
Tertiary dichroic mirror (overcoated Ag) 1.0%
Secondary central bevel (scattered environmental radiation) 0.2%
Secondary support vanes: - reflected in primary 0.1%
- direct view 0.0%
TOTAL: 3.4%

This should more than double the efficiency (speed) of UKIRT in the thermal IR, even relative to the model, and perhaps, since in practice the emissivity currently averages more like 12%, almost tripling it.

Emissivity measurements

Various factors act to degrade (increase) the emissivity. The actual coatings of primary, secondary and tertiary (dichroic) mirrors can deteriorate because of the corrosive effects of atmospheric gases and of particulate contamination. The secondary normally rather slowly as it faces downwards and accumulates less of the latter, but both primary and tertiary face upwards and can accumulate potentially corrosive material quite rapidly. Dust on the primary is removed from time to time, currently by cleaning with a jet of CO2snow. Cleaning the tertiary is currently a contentious issue (see below).

In order to plan such activities the emissivity of UKIRT is systematically monitored using the spectrometer CGS4. This is done by measuring background signal in a narrow wavelength range at 3.495 microns, where there are no atmospheric absorption features and therefore essentially no atmospheric background signal, so that any background observed arises in the telescope. The signal with the telescope pointed at clear zenith sky is measured and then compared with that seen when the dome and mirror covers are closed. The second signal approximates to that of a black body at the same temperature, i.e. the signal that would be seen if the telescope had 100% emissivity. The ratio of the two signals is therefore the emissivity of the telescope plus instrument window.

These measurements are nearly automatic and take only a few minutes. A measurement of the emissivity it routinely carried out at the end of most nights when the spectrometer is in use (bright dawn skylight has been shown to have no effect on the background level at this wavelength).

A reflectometer is also being used to monitor changes in the primary mirror and dichroic coatings directly, to validate and check the Emissivity Model. 

Results and UKIRT’s emissivity history

As part of the Upgrades Programme, on 31 December 1996 UKIRT was equipped with a dichroic with a silver-dielectric (Ag/di) coating. The emissivity shortly after this was ~14%. Late that year the emissivity after cleaning the primary mirror was around 16%, and by April 1998 it was 19% after cleaning.

The primary was aluminised on 30 May 1998. This produced little change in the system emissivity from the April measurement. Suspicion accordingly fell on the dichroic coating, and in August 1998 the dichroic was checked against a substrate with a (stable) gold coating. This revealed a difference in emissivity of around 7% attributable to deterioration of the Ag/di dichroic coating. The substrate with the old coating was replaced with one of similar age which had been in storage; the system emissivity was thereby reduced from ~19% to 12.8%, implying a telescope emissivity of ~11.8%, quite close to the model prediction.

Evidently the first dichroic coating had deteriorated quite sharply, to a component emissivity ~6% after nearly a year, ~9% after 15 months and perhaps 10% after 20 months. This was suspected to be a result of washing, which had been regularly carried out according to manufacturer’s instructions, as the stored dichroic had evidently not deteriorated.

The replacement coating was left rigorously unwashed. However in 1999 and into 2000 the emissivity was again creeping into the high teens of percent. A newly-coated replacement secondary had been installed in June and the reflectometer measurements showed that the primary mirror had not deteriorated to anything like this degree. A test on the dichroic revealed that it was once again the culprit and a replacement was installed on 8 February 2000. This reduced the overall emissivity from about 16% to ~10.5% (corresponding to a telescope emissivity of around 9.5%, in excellent agreement with the model).

Recently, the emissivity has been around the 18-19% mark. 

The following plot shows how the emissivity of UKIRT has changed between early 1999 and the summer of 2003 (just before the aluminizing shutdown in 2003). The trend indicates that the mirror has been slowly deteriorating and that has been increasing the emissivity at a rate of 1.6% per year. The lower envelope is actually the thing to look at, since there may be points affected by weather, and these points will overestimate the emissivity. At certain points, sharper increases can be seen in the emissivity, over a period of a few months. This is due to deterioration in the dichroic coatings and this contributes 0.83% per month to the emissivity.

A web page with the history of UKIRT’s measured emissivity can be found here.
UKIRT’s current overall emissivity, including cryostat window = 13.0% (25 Sept 2004)